Groundwater provides the largest source of usable water storage in the United States—accounting for about twenty percent of the world's fresh water supply. Groundwater is subsurface water that fully saturates pores or fractures in soil and rock formations. For example, a unit of water-bearing permeable rock, or unconsolidated sediment, is called an aquifer when the rock formation can yield a usable quantity of water. Aquifers are replenished by the seepage of precipitation that falls on the land above the aquifer but also can be artificially replenished. However, since groundwater is out of sight, locating usable subsurface water is difficult.
In developing countries—or other regions where water is scarce and where irrigation is essential for crops—accurately finding, managing, and preserving groundwater resources is important to avoid costly drilling work. Furthermore, when the groundwater is to be used for drinking water, it is important to identify groundwater of low salinity. To locate groundwater accurately and to determine the depth, quantity, and quality of the groundwater, several techniques must be used, and a target area must be thoroughly tested and studied to identify hydrologic and geologic features important to the planning and management of the water resource.
Existing systems used for groundwater exploration rely on electrical or electromagnetic methods to determine the distribution of electrical resistivity in the subsurface. The resistivity of soil or rock depends on a number of factors including, for example, the porosity, salinity of the pore fluid, water saturation (the degree to which the pore space is filled with water), and on the clay content. Small amounts of clay have a disproportionate effect on lowering the observed resistivity.
Since the resistivity of a given region of the subsurface depends on so many factors, working backwards from resistivity measurements to identify a good freshwater aquifer is highly problematic. The usefulness of resistivity mapping methods for groundwater largely depends on determining the resistivity-depth profile at a location where an aquifer has been identified and then extrapolating its extent through resistivity or transient electromagnetic (TEM) soundings taken laterally away on the surface.
Magnetic resonance sounding (MRS) can be used as a surface measurement tool to investigate the existence, amount, and productiveness of subsurface water. As a variant of nuclear magnetic resonance (NMR), MRS detects total water content of soils and rocks by exciting protons in the water molecules with an externally applied magnetic field at the Larmor frequency. The externally applied magnetic field is usually generated by passing a current at the Larmor frequency, supplied by a current generator, into a single, or a multi-turn, coil of wire on the surface (typically referred to as a loop transmitter). The application of this magnetic field causes the protons in the water molecules to align with the applied field direction throughout the volume of the ground where the magnetic field is large enough to influence the protons. A weak field will only align protons close to the source; whereas, a stronger field will align protons to a greater depth. Accordingly, MRS is accomplished by repeated measurements at gradually increasing strengths of the applied exciting field.
When the applied field is turned off, the aligned protons then precess (or “wobble”) around the direction of the Earth's static magnetic field. The precessing proton magnetic moments produce a secondary magnetic field at the Larmor precession frequency that is, in turn, detected by a sensitive magnetic field sensor back on the surface. This magnetic sensor is usually another single or multi-turn loop of wire commonly called a loop receiver. The precession frequency (the Larmor frequency) is a precise function of the value of the local static magnetic field and is known to high accuracy though measurement of the static magnetic field at the sounding site. The amplitude of the Larmor secondary field at the instant cessation of the inducing field is directly proportional to the water content in the volume of the subsurface influenced by the primary inducing field. The decay of the induced Larmor secondary field, called the relaxation time, provides information on the pore structure of the rock or soil formation containing the water.
MRS is sensitive to the spin of the nuclei under investigation (e.g., the spin of the nuclei of hydrogen protons of water molecules—the physical parameter which distinguishes water from any other material in the subsurface) in the presence of a static magnetic field (e.g., the Earth's magnetic field). Nuclei of the same species in different chemical environments (e.g., the hydrogen nuclei in water, benzene, or cyclohexane) possess different resonance frequencies. Therefore, MRS facilitates a direct search for not only groundwater, but also hydrocarbons and some other mineral deposits.
Additional details regarding the MRS method and application for the study of groundwater are discussed in related articles entitled “A review of the basic principles for proton magnetic resonance sounding measurements,” Legchenko, A., et al., Journal of Applied Geophysics 50 (2002) 3-19; “Nuclear magnetic resonance as a geophysical tool for hydrogeologists,” Legchenko, A., et al., Journal of Applied Geophysics 50 (2002) 21-46 and “MRS: A new geophysical technique for groundwater work,” Roy, J., The Leading Edge (October. 2009) 1226-1233, which references are hereby incorporated by reference in their entireties. The most recent review with a description of new excitation pulse sequences for determining accurate relaxation times is presented in: “A Review of the Principles and applications of the NMR Technique for Near-Surface Characterization, Behroozmand et al., Surv. Geophys., Springer, September 2014, which reference also is herein incorporated by reference in its entirety.
Electromagnetic (EM) measurements of ground resistivity are made by inducing currents, usually called Faraday currents, to flow in the subsurface by producing a changing magnetic field from a transmitter loop on the surface. The transmitter loop carries a time changing current that produces a time-varying magnetic field. This time changing current can be a continuous sinusoid of a predetermined frequency. Sinusoidal measurements can be measured in the frequency domain (called a frequency domain measurement). Additionally, the time changing current can also be a pulse of current, which typically appears as a square wave or a half-sine pulse. Pulse currents can be measured in the time domain (called a time domain measurement).
The induced currents in the ground are proportional to the ground conductivity and produce a secondary magnetic field that is measured with a separate receiver. In a central loop configuration for TEM, the transmitter loop is a circular loop, and the receiver is a smaller concentric loop for detecting the secondary field. Such a configuration can operate in the time domain and measurements of the decaying or transient fields after the termination of the pulse are related to the distribution of electrical resistivity in the formations beneath the transmitter-receiver system. A review of transient electromagnetic methods is presented in “Time Domain Electromagnetic Prospecting Methods,” Nabighian, M. N., et al., Ed., Electromagnetic Methods in Applied Geophysics, Vol. 2, Parts A and B, Soc. Expl. Geophysics (1991) and “Use of Electromagnetic Methods for Groundwater Studies,” McNeill, J. D., Ward, S. H., Ed., Geotechnical and Environmental Geophysics, Vol. 1: Review and Tutorial, Soc. Expl. Geophysics (1990), which references are hereby incorporated by reference in their entireties.
A short pulse of current in the TEM transmitter loop produces a time-varying magnetic field that induces a pattern of concentric currents in the ground that decays after the energizing pulse is terminated. The currents diffuse downwardly and radially outwardly from the transmitter loop, and their rate of decay and the shape of the measured secondary field transient depend on the vertical distribution of electrical resistivity in the ground beneath the transmitter.
The combination of MRS and resistivity mapping removes many ambiguities of conventional electrical-/electromagnetic-based groundwater exploration tools—particularly by identifying the salinity of an aquifer detected by MRS. To date, few surveys have addressed this problem by conducting separate MRS and resistivity mapping measurements at the same site but with different equipment (e.g., “Application of the integrated NMR—TDEM method in groundwater exploration in Israel,” Goldman, M., et al., Journal of Applied Geophysics 31 (1994) 27-52; “Aquifer characterization using surface NMR jointly with other geophysical techniques at the Nauen/Berlin test site,” Yaramanci, U., et al., Journal of Applied Geophysics 50 (2002) 47-65; “Surface NMR sounding and inversion to detect groundwater in key aquifers in England: comparisons with VES-TEM methods,” Meju, M. et al., Journal of Applied Geophysics 50 (2002) 95-111, which references are hereby incorporated by reference in their entireties). Goldman et al., in particular, presents a very succinct rationale for the importance of joint resistivity and MRS measurements in identifying water quality.
In areas where resources are limited (e.g., deployment in developing countries), a field system that combines both techniques in a common instrumentation package using common transmitters and receivers needs to be lightweight, low-power, and requires simple operation to drastically improve the efficiency of fresh groundwater exploration. Accordingly, a need exists for improved systems and methods for groundwater exploration in an effort to overcome the aforementioned obstacles and deficiencies of prior art systems.